1. Field of the Invention
This invention in general relates to a position control system for use in magnetic recording devices and more particularly to a system for providing radial positioning of a magnetic transducer head through recording and reproducing track positioning or control information along a pair of control tracks of a moving magnetic recording medium.
The invention is based on the realization that a track address command can be represented as a fixed time interval, and on the fact that radial displacement of a magnetic transducer head relative to a magnetic recording medium can be determined through measuring a time differential, where the duration of the interval varies in accordance with the extent of radial displacement.
2. Description of the Prior Art
The art of information storage has developed rapidly with constant improvements in the amount of information that can be stored in a particular media and in the time required for access to this stored information. For information that must be stored frequently, but not in a fixed sequence, the time for retrieval becomes of paramount importance. Retrieval of information stored at random can be excessively time consuming in certain applications. Therefore, apparatus for magnetic storage of data and servo information on cylinders and disks have been developed in the art to permit random access to recorded data by direct positioning of a magnetic transducer head over the portion of the cylinder or disk containing the information sought. Read/write transducers as well as control transducers are mounted in a head assembly and extend adjustably over the recording surface of the disk, the head being supported by an arm or linkage which is stepped radially by a linear motor. The disk rotates at generally high speeds and accurate positioning of the magnetic head in radial distance relative to the axis of rotation is imperative. During high speed rotation of the disk, thin air layers are created and follow the disk surface. A properly designed and mounted head system will "fly" on the air layers in close proximity with a disk surface.
Information is recorded in serial digital form along a plurality of concentric tracks on the magnetic recording surface of the disk. Each data transducer in the head assembly is positioned over a track on the recording surface of the disk for a given linear position of the head. Selection of an individual track is accomplished by means of, for example, a servo system including one or more servo transducers. The tracks may be divided into sectors such that only a segment of the track may be selected for reading or recording. It is apparent that the narrower the tracks are and the smaller the tolerance permitted between tracks, the higher the density and the greater the amount of information that can be stored on a given disk surface. Consequently, positioning systems of the type utilized, for example, in data disk drives must be highly reactive in positioning a magnetic head and moreover must not only have a fast access time enabling retrieval of stored data information, but must also be able to accurately position the transducer head adjacent any given one of a multitude of closely spaced tracks.
Prior art magnetic track detection and measuring transducers are known which are sufficiently accurate for use with high velocity head positioning systems of the type under discussion. An example of such a system is illustrated and described in the aforementioned U.S. Pat. No. 3,686,649. The patent discloses a technique for precise control of magnetic recording transducers adjacent the face of a rapidly rotating disk in a magnetic memory system. A series of information or data transducers are rigidly coupled to a position sensing transducer and mounted so that the assembly is adjustably movable in a radial direction relative to the disk. A number of data tracks in an information zone extends circumferentially around the disk. Concentric with this information zone is a control or servo track used for positioning the position sensing transducer.
Within the control channel, there are periodic magnetic transitions in lines skewed alternately clockwise and counterclockwise from the disk radius so as to form a series of interlaced trapezoidal regions with the transition between adjacent regions being diagonal to the circumferential extent of the control channel. As the position sensing transducer is shifted radially, the time interval between sensing of successive magnetic transitions varies and this sensed time interval is compared with a reference signal to give a position error signal used for servo control of the position sensing transducer.
The magnetic position sensing transducer is provided with a trapezoidal core gap for recording the positioning signals in the control track. In the embodiments illustrated in the patent, the trapezoidal gap is either completely free of magnetic material or a trapezoidal shunt may be provided within the gap and spaced apart from the transducer pole pieces for simultaneously produced, spaced apart diagonal magnetic transitions along the control track.
The position sensing transducer and data transducers are mechanically linked together and supported on an arm, the latter being actuated by a linear motor in response to error signals applied thereto by servo control circuitry.
According to the system disclosed, initially the entire control channel is polarized in one direction following which the separate write transducer is moved along the control channel to reverse the polarity in selected trapezoidally-shaped areas.
One disadvantage encountered with this system, however, is the fact that in addition to the servo "read" transducer used for reproducing position control information corresponding to a predetermined servo control pattern recorded on the control channel, a separate servo "write" transducer not ganged to the read transducer and non-unitary with the transducer support assembly, is to be used for recording such control patterns.
The undesirability of such a system utilizing separate read and recording transducers which individually read and write positioning information, emphasizes the desirability to provide a system in which track positioning information is both recorded and, subsequently, reproduced by the same positioning transducers located in a single transducer head assembly.